Differences from Upstream

Local vs. Remote Repositories

Tycho should respect the state from which the maven build was initiated. Launching a maven build through XMvn (xmvn/mvn-build/%mvn_build) should cause the resolution of Fedora Tycho, and depending on whether an online/offline functionality is set, Tycho will resolve its artifacts accordingly. That is :

When maven is running through XMvn, and offline mode is set, Tycho will resolve its dependencies using only local resources. (eg. reactor cache, system local p2 repository). When maven is running through XMvn, and offline mode is not set, Tycho will first attempt to resolve its dependencies using local resources. If a resource cannot be found locally, Tycho will check remote repositories and place resources into the reactor cache as needed. In this case, a look through the logs can determine what packages were downloaded remotely.

Local OSGi bundle resolution

Tycho is generally expected to find all OSGi bundles installed on the system in accordance with the Fedora Package Guidelines. These locations will often include : 'usr/share/java', 'usr/lib*/eclipse', 'usr/share/eclipse/dropins' relative to some specified root (eg. '/', or '/opt/my/scl/' )

Target Platform Configuration

By default, upstream's offering of Tycho will attempt to resolve dependencies for every environment defined for the pom's target-platform-configuration. So for example, if your plugin needs Eclipse SWT, then the build will need to have the SWT fragment for every environment defined.

With access to remote p2 repositories, we could build for any environment, but in Fedora we must only use packages already packaged, and for that particular environment. As a result building for multiple environments at once isn't easily achievable, and in some cases the environment isn't supported by Fedora at all. This can cause all sorts of failures when running a build due to environment-specific dependencies not being available.

To get around this, Fedora's Tycho will (by default) set the target platform environment to be the same as the currently running system. In some cases, certain plugins being built are tied to a certain environment, and if this does not match the system's running environment, they will fail to build. Ideally upstream should have build profiles which allow customization of what is being built. As a last resort, Fedora Tycho has the flag tycho.local.keepTarget. With this flag enabled, Fedora Tycho will behave as an upstream Tycho with regard to target platform configuration. That is, Tycho will set the target platform environment as specified in the project poms.

If you are using tycho.local.keepTarget then you may find that your build is unnecessarily assembling products for platforms that you don't care about. In this case you can use the director.running.env.only flag to make tycho's p2-director plug-in only assemble products for the running target environment -- all other configured environments are omitted. This saves a lot of build time for large products on slow build environments such as 32bit ARM.

Multiple Tycho Versions Specified

As of Tycho 0.16.0, when a project depends upon a version of Tycho that doesn't match the system's version, it will fail. Upstream has implemented this to avoid scenarios where a project might specify multiple versions and fail at runtime with cryptic error messages. This seems to be triggered even when using a lower version and expecting any higher available version to be resolved. Fedora Tycho 0.16.0 has added the property skipTychoVersionCheck to disable this check. As of Tycho 0.18.1, Tycho version checking is ignored by default, so no action needs to be taken. In the event that version checking is desired, the tycho.enableVersionCheck property may be set. (See http://git.eclipse.org/c/tycho/org.eclipse.tycho.git/commit/?id=305eb576d58f1adf8c8f80fee8bdca49da196816 for more details)

Bootstrap Building

The building of Tycho in Fedora is different from most builds. You may have noticed that upstream Tycho actually uses an pre-existing Tycho instance to build itself. This is because Tycho contains OSGi bundles as reactor projects. As a result Tycho has a dependency on itself. In Fedora we're able to break this chain if necessary to build Tycho cleanly when a build does not already exist (none in the buildroot).

To find all Tycho components needed for the build, we look for any components that use ${tychoBootstrapVersion} as their version and add them as well as any dependencies they have on other Tycho components. These are all clearly defined in the pom.xml files and we can then picture a dependency graph of these components. The graph can be generated using the tycho-bootstrap-reader maven plugin.

In addition to this we add any components that Tycho seems to use at runtime. TychoOsgiRuntimeArtifacts.java as well as components.xml (tycho-maven-plugin) contain such components. Certain Tycho projects are of an 'eclipse-test-plugin' type, but despite the fact that we can skip tests, resolution and compilation phases are still run. To avoid having to build tycho-surefire-plugin and it's various dependencies for runtime availability, we can modify the actual lifecycle mapping for our bootstrapped Tycho to omit running tycho-surefire-plugin at runtime.

Once we've discovered and ordered the necessary components we perform the following steps to create a bootstrapped Tycho build :

Build all required OSGi bundles in the tycho-bundles folder using a combination of javac and jar to properly package them. Where certain Tycho OSGi bundles depend on other Tycho OSGi bundles, or external OSGi bundles we set the classpath appropriately.

Place the built OSGi Tycho bundles into the reactor cache as if we had run mvn org.apache.maven.plugins:maven-install-plugin:${install-plugin.version}:install

Once this is completed, we may perform a real build ensuring that Tycho will resolve itself entirely from the reactor cache we have built up.

Previously, when building a newer Tycho with the same version as the previous, we would first build an intermediary version and then the final version to avoid any cyclic dependencies. Tycho checks reactor projects first when resolving dependencies, so attempting to build a particular version while depending on an identically versioned Tycho would cause a cycle ( See http://dev.eclipse.org/mhonarc/lists/tycho-user/msg02599.html ) . Now we simply modify the version metadata of the Tycho we are depending upon to be different than the one we are attempting to build, thus eliminating the need for an intermediary build. See Fedora Tycho Commit 369a33 for more detail on how this occurs.

Missing Eclipse Parent POM

Many Eclipse projects building at the foundation use an eclipse parent pom to reference various things that most projects would have in common. While this is encouraged for upstream builds, Eclipse does not currently ship this parent pom in Fedora and references to it should be removed. See http://wiki.eclipse.org/Maven/Parent_POM for more details.

Build Qualifiers

By default tycho-packaging-plugin will generate build qualifiers using a timestamp from the time of the build. It is however also possible to configure the packaging plugin to set the build qualifier for artifacts based on a custom provider. One common timestamp provider uses JGit to get the last modification time for a sub-project (in Git version control). This capability requires that a Git repository exist, but most projects ship their sources with the repository metadata removed. The error seen in these cases is often 'java.lang.IllegalArgumentException: One of setGitDir or setWorkTree must be called.' .

To avoid this issue, packagers must decide whether they wish to include some git repository metadata in the sources (by creating a shallow clone of a repository) or to fall back to default functionality (often by disabling the customized declaration of tycho-packaging-plugin).